Bluetooth: controller: Fix default Tx buffers

Fix the default Tx buffers to 3. While the first Tx-ed
buffer generates the HCI Number of Completed Packets Event,
the controller needs to have 2 additional Tx buffers queued
so that the second Tx PDU has the More Data (MD) bit set so
as to have the connection event to continue to transmit any
additional Tx buffers that the Host will enqueue in the same
connection event.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>